We say "who has a black car?", and the answer is "John has one", or "John and Paul have silver cars". Why do we say "who has"??? Because we obviously do not know the answer, is it one person or more than one?? The same rule applies for the verb to be. Consquently we ask "Who was at the party", and the answer can be singular or plural. Of course we expect a lot of people to be at the party, but what if no one shows up? Then we say: no one was at the party.